Malfunction-detecting method in injection molding machines

ABSTRACT

This invention provides a method which makes it possible to reliably detect malfunctions during the ejecting step without rendering the injection molding machine more complicated. An ejector pin is incorporated in the movable die. This ejector pin is connected, via an ejector plate, a connecting rod, a connecting plate and a feed screw, with a servo motor. The force for ejecting the ejector pin pushing a molded product is measured from the driving torque of a servo motor. In this invention, the relationship between driving torque and elapsed time from the initiation of ejection of the molded product is stored as a reference pattern. The tolerance limit and monitoring interval of driving torque are preset relative to the reference pattern. When the value of driving torque falls outside the tolerance limit during the ejecting step, it is judged as a malfunction, and an alarm is generated to stop the machine in a safe state.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-210405, filed Jul.11, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a method of detecting malfunctionsduring the ejecting step of removing molded products from a mold in amotorized or hydraulic injection molding machine.

[0003] Injection molding machines are generally operated as follows.First of all, the mold thereof is clamped, and then, a melted materialis injected from the injection unit into a mold. Then, after the resinis sufficiently freezed, the releasing of mold is performed. After thereleasing of mold, an ejector pin incorporated inside a movable die isallowed to protrude from the inner wall of the die so as to push themolded product out of the die. With respect to this ejecting step, ithas been conventionally practiced such that the ejecting speed, ejectingforce and stroke of the ejector pin, etc. are set in advance by theoperator. However, with regard to the evaluation of the result ofproduct removal from the mold, the measures that have been takenaccording to the prior art were simply to confirm the ejecting stroke ofejector pin, thus omitting any other automatic evaluation of the resultof removal of the molded product.

[0004] As mentioned above, according to the control system of theconventional injection molding machine, if only the advancing movementof the ejector pin up to a preset stroke can be confirmed in theejecting step, the ejecting step is judged as being normally finished,thus enabling the injection molding machine to start the next moldclamping step.

[0005] However, there is occasionally a case where the ejector pin ispierced into a molded product, thereby leaving the molded productadhered to and unremoved from the surface of the mold. In such a case,it is impossible according to the conventional control system torecognize the existence of the molded product on the front surface ofmold. Therefore, if the next mold clamping operation is performed undersuch an abnormal condition, the molded product is caught between thepair of dies, thus leading to damage of the mold.

[0006] With a view to prevent such an accident, there has been proposeda malfunction-detecting system which is based on image processing. Thissystem however is accompanied with drawbacks that the installation costthereof is high, and that a delicate adjustment of illumination, etc. isrequired for obtaining a clear image, thereby making it troublesome inhandling the system.

BRIEF SUMMARY OF THE INVENTION

[0007] This invention has been accomplished in view of overcoming theproblems accompanied with the conventional malfunction-detecting methodin the ejecting step of the conventional injection molding machine.Therefore, an object of this invention is to provide amalfunction-detecting method which makes it possible to reliably detectmalfunctions in the ejecting step without rendering the injectionmolding machine more complicated.

[0008] Namely, this invention provides a malfunction-detecting method inelectric injection molding machines, in which a molded product isremoved from a mold by using an ejector pin in an ejecting step, themethod comprising the steps of:

[0009] storing in advance a relationship between time and torque of anejector pin driving motor as a reference pattern, the relationshiphaving been obtained from good results in the removal of the moldedproduct;

[0010] presetting a tolerance limit of the reference pattern withrespect to the value of the torque; and

[0011] monitoring a pattern of the torque, relative to time, during theejecting step, and generating an alarm at the time when the value of thetorque falls outside the tolerance limit.

[0012] The malfunction-detecting method of this invention to detect amalfunction in an injection molding machine is based on the followingprinciple. Namely, when, for instance, the ejector pin is pierced into amolded product, thus leaving the molded product adhered to and unremovedfrom the surface of the mold in the ejecting step after the releasing ofthe mold, the torque of the ejector pin-driving motor is of an abnormalvalue. Therefore, when a pattern indicating this abnormal value oftorque is compared with the normal torque pattern, the occurrence of amalfunction in the ejecting step can be easily detected.

[0013] Further, even when the cavity of the mold is under-filled orover-filled, part of a molded product may be adhered to and unremovedfrom the surface of the mold. Even in this case, the torque of theejector pin driving motor is of an abnormal value, so that theoccurrence of a malfunction in the ejecting step can be reliablydetected.

[0014] Preferably, a time interval for monitoring the torque pattern ofthe ejector pin driving motor relative to the time in the ejecting stepis possible to be preset by the operator.

[0015] By doing so, it becomes possible for the operator to optionallypreset in advance, as a monitoring interval, a region where the changesof torque are relatively mild in the torque pattern in the normaloperation of the ejecting step, or a region where a pattern peculiar tothe generation of various kinds of malfunction is likely to bemanifested. As a result, the accuracy in detection of a malfunction canbe improved.

[0016] Incidentally, the aforementioned malfunction-detecting method canalso be applied to a hydraulic injection molding machine in the samemanner as that of the electric injection molding machine. In this casehowever, the pattern to be monitored would be a pattern of the hydraulicpressure of the ejector pin-driving hydraulic pump instead of thepattern of torque of the ejector pin-driving motor.

[0017] Incidentally, the aforementioned “time” refers to the timeelapsed from the start of the ejecting operation. Instead of using this“time”, either the position of the ejector pin, or the rotation angle ofmotor for driving the ejector pin can be also utilized.

[0018] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0019] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently embodimentsof the invention, and together with the general description given aboveand the detailed description of the embodiments given below, serve toexplain the principles of the invention.

[0020]FIG. 1 is a schematic diagram illustrating a electric injectionmolding machine to which the malfunction-detecting method according tothis invention is applied;

[0021]FIG. 2 is a graph illustrating one example of the referencepattern of “time to driving torque” in the ejecting step;

[0022]FIG. 3 is a graph illustrating one example of a tolerance limit ofthe driving torque to be set in the ejecting step;

[0023]FIG. 4 is a graph illustrating one example of a monitoringinterval of the driving torque to be set in the ejecting step; and

[0024]FIG. 5 is a graph illustrating one example of the pattern of “timeto driving torque” when a malfunction has occurred in the ejecting step.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 1 schematically illustrates a motorized injection moldingmachine to which the malfunction-detecting method according to thisinvention is applied. In FIG. 1, the reference number 1 represents amolded product, 2 represents a fixed die, 3, represents a movable die, 4represents an ejector pin, 6 represents a movable platen, 15 representsa servo motor (driving motor), and 20 represents a control system.

[0026] The movable die 3 is held on the front surface of the movableplaten 6. The movable die 3 is provided at the central axis thereof witha through-hole, into which the ejector pin 4 is slidably incorporated.An ejector plate 5 is interposed between the movable die 3 and themovable platen 6, and is held movably in the axial direction thereof infront of the movable platen 6. The rear end of the ejector pin 4 isfixed to a central portion of the ejector plate 5.

[0027] The movable platen 6 is provided at the central axis thereof witha through-hole, into which a connecting rod 7 is slidably incorporated.On the rear side of the movable platen 6, there is disposed a connectingplate 8 which is held movably in the axial direction thereof behind themovable platen 6 via a supporting rod 11 and a feed screw 12. The rearend of the connecting rod 7 is fixed to a central portion of theconnecting plate 8. This-feed screw 12 is connected, via a gear wheel,with the servo motor 15. The position of the connecting plate 8 (and theconnecting rod 7) can be measured on the basis of the output of arotation angle detector 16 attached to the servo motor 15.

[0028] According to this injection molding machine, after the setting ofthe resin filled in a cavity between the fixed die 2 and the movable die3, the movable die 3 is retracted to thereby release the mold. Themolded product 1 is left adhered onto the front surface of the movabledie 3. Thereafter, by making use of the servo motor 15, the connectingrod 7 is actuated so as to protrude the ejector pin 4 from the innerwall of the movable die 3. As a result, the molded product 1 is pushedout of the mold and recovered.

[0029] The distance the ejector pin 4 is ejected to push and recover themolded product 1 can be measured from the output of the rotation angledetector 16. On the other hand, the force to protrude the ejector pin 4can be measured from the driving torque of the servo motor 15(therefore, from the driving current).

[0030] The control system 20 for driving the ejector pin 4 isconstituted by an ejector control amplifier 21, a sensor input unit 22,a storage and arithmetic unit 23, an MMI/F (man-machine interface) 24,and a control output unit 25. The ejector control amplifier 21 functionsto take up the driving torque and rotation angle of the servo motor 15employed for driving the ejector pin 4 and also to control the operationof the servo motor 15. The sensor input unit 22 functions to send thedata of the driving torque and rotation angle of the servo motor 15 thathave been transmitted thereto from the ejector control amplifier 21 tothe storage and arithmetic unit 23. This storage and arithmetic unit 23functions to determine the driving conditions of the servo motor 15 onthe basis of the instruction that has been input via the man-machineinterface 24 by the operator, and to send a command to the controloutput unit 25. This control output unit 25 functions to send a controlsignal to the ejector control amplifier 21 to thereby control the servomotor 15.

[0031] Next, the malfunction-detecting method will be explained that isemployed to remove the molded product 1 from the movable die 3 byejecting the ejector pin 4 in the electric injection molding machineshown in FIG. 1.

[0032] First of all, the relationship of the time to driving torque whengood result has been achieved in the removal of the molded product, isstored as a reference pattern. FIG. 2 shows one example of such areference pattern. Incidentally, in FIG. 2, the starting point of anejecting operation lies at the origin of abscissa (time-axis).

[0033] Then, as shown in FIG. 3, the tolerance limit of the drivingtorque relative to the reference pattern is preset. When the value ofthe driving torque falls outside the aforementioned tolerance limit inany moment during the ejecting step, it is judged as a malfunction andtherefore, an alarm is generated to call the operator's attention to themalfunction, thereby making it possible to stop the machine in a safestate.

[0034] Incidentally, as shown in FIG. 4, it is possible for the operatorto optionally preset a time interval (i.e. monitoring interval) on thetime-axis which enables the aforementioned malfunction-detectingfunction to work in the ejecting step. By doing so, it becomes possiblefor the operator to optionally preset in advance, as a monitoringinterval, a region where the changes of torque are relatively mild inthe torque pattern in the normal operation of the ejecting step, or aregion where a pattern peculiar to the generation of various kinds ofmalfunction is likely to be manifested. As a result, the influence ofnoise can be minimized, and the accuracy in detection of a malfunctioncan be improved.

[0035]FIG. 5 shows one example of the pattern of “time to drivingtorque” when a malfunction has been detected in the ejecting step.According to this example, when the ejector pin 4 is pierced through themolded product 1, an abnormal peak can be recognized in the drivingtorque.

[0036] Incidentally, although the foregoing explanation is limited tothe case where this invention is applied to the electric injectionmolding machine, the malfunction-detecting method of this invention canbe also applied to a hydraulic injection molding machine in the samemanner as explained above. In this case however, the pattern to bemonitored would be a pattern of the hydraulic pressure of the ejectorpin-driving hydraulic pump instead of the pattern of torque of theejector pin-driving motor.

[0037] Incidentally, in FIGS. 2 to 5, the abscissa thereof may representthe position of the connecting plate 8 instead of the time. In thiscase, as shown in FIG. 2, the reference pattern of the driving torque ofservo motor relative to the position should be stored, and as shown inFIG. 3, the tolerance limit of the driving torque relative to thisreference pattern should be preset. Therefore, if the interval on theposition-axis which enables the aforementioned malfunction-detectingfunction to work in the ejecting step, is preset in advance by theoperator as shown in FIG. 4, the generation of an abnormal peak in thedriving torque can be readily detected as shown in FIG. 5.

[0038] As explained above, it is possible, according to the method ofthis invention, to reliably detect a malfunction in the ejecting step ofthe injection molding machine without rendering the injection moldingmachine more complicated.

[0039] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A malfunction-detecting method in electricinjection molding machines, in which a molded product is removed from amold by using an ejector pin in an ejecting step, said method comprisingthe steps of: storing in advance a relationship between time and torqueof an ejector pin driving motor as a reference pattern, saidrelationship having been obtained from good results in the removal ofthe molded product; presetting a tolerance limit of said referencepattern with respect-to the value of said torque; and monitoring apattern of said torque, relative to time, during the ejecting step, andgenerating an alarm at the time when the value of said torque fallsoutside said tolerance limit.
 2. The malfunction-detecting methodaccording to claim 1, wherein it is possible for an operator to preset atime interval for monitoring the torque pattern relative to the time inthe ejecting step.
 3. A malfunction-detecting method in hydraulicinjection molding machines, in which a molded product is removed from amold by using an ejector pin in an ejecting step, said method comprisingthe steps of: storing in advance a relationship between time andhydraulic pressure of an ejector pin-driving hydraulic pump as areference pattern, said relationship having been obtained from goodresults in the removal of the molded product; presetting a tolerancelimit of said reference pattern with respect to the value of saidhydraulic pressure; and monitoring a pattern of said hydraulic pressurerelative to time during the ejecting step, and generating an alarm atthe time when the value of said hydraulic pressure falls outside saidtolerance limit.
 4. The malfunction-detecting method according to claim3, wherein it is possible for an operator to preset a time interval formonitoring the hydraulic pressure pattern relative to the time in theejecting step.
 5. A malfunction-detecting method in electric injectionmolding machines, in which a molded product is removed from a mold byusing an ejector pin in an ejecting step, said method comprising thesteps of: storing in advance a relationship between the position ofejector pin or the rotational angle of an ejector pin driving motor andtorque of the ejector pin driving motor as a reference pattern, saidrelationship having been obtained from good results in the removal ofthe molded product; presetting a tolerance limit of said referencepattern with respect to the value of said torque; and monitoring apattern of said torque, relative to said position or said rotationalangle during the ejecting step, and generating an alarm at the time whenthe value of said torque falls outside said tolerance limit.
 6. Themalfunction-detecting method according to claim 5, wherein it ispossible for an operator to preset an interval for monitoring the torquepattern relative to said position or said rotational angle in theejecting step.
 7. A malfunction-detecting method in hydraulic injectionmolding machines, in which a molded product is removed from a mold byusing of an ejector pin in an ejecting step, said method comprising thesteps of: storing in advance a relationship between the position ofejector pin and hydraulic pressure of an ejector pin-driving hydraulicpump as a reference pattern, said relationship having been obtained fromgood results in the removal of the molded product; presetting atolerance limit of said reference pattern with respect to the value ofsaid hydraulic pressure; and monitoring a pattern of said hydraulicpressure, relative to said position, during the ejecting step, andgenerating an alarm at the time when the value of said hydraulicpressure falls outside said tolerance limit.
 8. Themalfunction-detecting method according to claim 7, wherein it ispossible for an operator to preset an interval for monitoring thepattern of said hydraulic pressure relative to said position in theejecting step.